Wind-driven electric power generation is non-polluting and is thus considered to be environmentally friendly. This form of power generation is further desirable in view of the utilization of a renewable source of energy, the wind. A commonly cited concern of power system operators is that wind turbines that use electronic interfaces, such as doubly-fed induction generators or generators with a full converter interface, do not respond to correct abnormal frequency (e.g., underfrequency) conditions as do directly connected machines. For example, during system frequency reductions, directly connected machines in a self-acting manner can convert a portion of a shaft kinetic energy to supplemental electric power.
Directly connected generator machines contribute to frequency and/or voltage regulation by producing an electromagnetic torque that opposes any change in rotational speed and the torque is proportional to the product of the inertia and the rate of change of the shaft rotational speed. This incremental torque (and incremental electric power) is commonly referred as the “inertial response” of the machine to a change in system frequency.
Wind turbines that have electronic interfaces to a power grid are inherently insensitive to frequency changes and do not produce any response to frequency changes unless they are programmed to do so. One approach is described in PCT application No. WO 03/023224 A1, filed on 20 Mar. 2003, titled “System For Using Energy Stored In The Mechanical Inertia Of The Rotor Of A Wind Turbine”, which describes a system for using turbine mechanical inertia for dynamic stability and frequency control. The system purportedly uses a fixed frequency reference and the derivative of frequency (Δf/Δt) to calculate a supplemental torque and power output of the system. Derivative terms in control systems are generally subject to noise that can affect performance. Additionally, a fixed reference may lead to difficulties where the turbine control is desired to track the normal fluctuations in utility frequency without undue supplemental torque or power interactions. Another approach is described by U.S. Pat. No. 7,345,373, titled “System And Method For Utility And Wind Turbine Control, filed Nov. 29, 2005. The approach described in this US patent requires burdensome computation of an internal reference frame and a determination of a concomitant frequency of rotation ωi for the internal reference frame and further requires that the supplemental power signal be modified as a function of the internal reference frame and the measured frequency to purportedly reduce the effects of noise that would result from using a derivative signal of the inertial response, as proposed by the above-cited PCT application.
Therefore, there is a need to overcome the above-mentioned issues and to provide apparatus and/or control techniques conducive to improvements in connection with frequency regulation of wind turbines.